Field of the Invention
The disclosed technology generally relates to semiconductor devices and more particularly to memory devices having a resistance switching element, and to methods of operating such memory devices.
Description of the Related Technology
Resistive random access memory (RRAM) has emerged as one of the most promising non-volatile memory candidates due to its simple structure and its potential for low-power operation. Recently, memory arrays having cross-bar architectures have attracted considerable attention. Memory arrays having cross-bar architectures, sometimes called “cross-bar arrays” or “cross-point arrays,” have a first plurality of conductive lines (e.g., bit lines) and a second plurality of conductive lines (e.g., wordlines) that cross the first conductive lines, and a plurality of memory cells formed at the intersections of the first and second conductive lines. Some cross-bar architectures can enable a high density of integrated memory cells. On the other hand, some cross-bar architectures pose several challenges. One of the challenges includes for example, suppressing leakage currents through unselected memory cells during operation of the cross-bar arrays, which can limit the size of the arrays. Another one of the challenges includes crosstalk and interference between memory cells during operation of the arrays.
To reduce the leakage currents through the unselected memory cells during operation, some cross-bar arrays employ selector devices. Such selector devices can be diodes, or more typically transistors. However, while selector devices can reduce leakage currents when included in the cross-bar arrays, some selector devices, such as transistors, can take up device footprint, which can limit the obtainable memory element density, which is a key technological as well as an economic consideration for practical implementation of standalone memory devices. Moreover, such transistor selector increases the complexity of fabrication of the array. In addition, some selector devices, such as semiconductor diodes, may not be able to provide sufficient current density for switching memory elements, which can exceed 1×106 A/cm2
The leakage currents flow through unselected memory cells during operation of the cross-bar arrays that are sometimes called “sneak current path,” which refers to the parasitic current path that appears through unselected memory cells when addressing a target selected memory cell in a crossbar memory array. Under some circumstances, such leakage currents can reduce the signal to noise ratio in reading the selected memory cell, which in turn increase the rate of error in reading.
Several self-rectifying (selectorless) cell attempts have been reported, but they exhibit either modest performance or difficult integration.
US2010/0315857 describes a resistance change memory including a first and a second conductive line in a cross bar configuration, a cell unit including a memory element and a rectifying element connected in series between the first and second conductive lines, and a control circuit which is connected to both of the first and second conductive lines. The control circuit controls a voltage to reversibly change a resistance of the memory element between first and second values. The rectifying element is a diode including an anode layer, a cathode layer and an insulating layer therebetween. The MIM diode structure described has the functionality of enabling current in one direction while suppressing it in the other direction. It is a disadvantage of this structure that no bidirectional rectification can be obtained.
US2012/0068137 discloses a switching device including a bipolar tunnelling layer sandwiched in between a first electrode and a second electrode, whereby the bipolar tunnelling layer includes a plurality of dielectric layers having different dielectric constants. It is a disadvantage of the disclosed switching device that it does not provide high enough currents for proper functioning of a memory cell having a resistance switching element.